Ultrasensitive and Highly Compressible Piezoresistive Sensor Based

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Applications of Polymer, Composite, and Coating Materials

Ultrasensitive and Highly Compressible Piezoresistive Sensor Based on Polyurethane Sponge Coated with Cracked Cellulose Nanofibril/Silver Nanowire Layer Shuaidi Zhang, Hu Liu, Shuaiyuan Yang, Xianzhang Shi, Dianbo Zhang, Chongxin Shan, Liwei Mi, Chuntai Liu, Changyu Shen, and Zhanhu Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00900 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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ACS Applied Materials & Interfaces

Ultrasensitive and Highly Compressible Piezoresistive Sensor Based on Polyurethane Sponge Coated with Cracked Cellulose Nanofibril/Silver Nanowire Layer

Shuaidi Zhang,1 Hu Liu,1,2* Shuaiyuan Yang,1 Xianzhang, Shi,1 Dianbo Zhang,1 Chongxin Shan,3 Liwei Mi,4 Chuntai Liu,1 Changyu Shen1 and Zhanhu Guo2

1Key

Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of Education; National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou, 450002, China 2Integrated

Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996 USA

3School

4

of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan 450052 China

Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou, Henan 450007 China

*: Correspondence authors E-mail addresses: [email protected] (H. L.), [email protected] (C. L.)

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ABSTRACT: With the rapid development of flexible wearable electronics, piezoresistive sensor with low detection limit and wide strain sensing range turn to be great challenges for its application in this field. Here, cracked cellulose nanofibril/silver nanowire (CA) layer coated polyurethane (PU) sponge was acquired through simple dip-coating process followed by pre-compression treatment. The electrical conductivity and mechanical property of the conductive CA@PU sponge could be effective tuned through changing the dip-coating number. As a piezoresistive sensor, the sponge exhibited the capability of detecting both small and large motion over a wide compression strain range of 0-80%. Based on the “crack effect”, the sensor possessed a detection limit as low as 0.2%, and the gauge factor (GF, GF=(R/R0)/ε, ΔR, R0 and ε represents the instantaneous resistance change, original resistance and strain applied, respectively) was as high as 26.07 in the strain range of 0-0.6%. Moreover, the “contact effect” enabled the sensor to be applicable for larger strain, and the GF decreased firstly and then became stable with increasing compression strain. In addition, frequency and strain dependent sensing performance were observed, demonstrating the sensor can response reliably to different applied frequency and strain. Furthermore, the sensor displayed exceptional stability, repeatability and durability over 500 cycles. Finally, the sensor could be applicable for the detection of various human bodily motion, such as phonation, stamping, knee bending and wrist bending. Most importantly, the sponge also exhibited great potential for the fabrication of artificial electronic skin. Herein, the conductive CA@PU sponge will undoubtly promote the development of high-performance flexible wearable electronics. KEYWORDS: PU sponge; Silver nanowire; crack; Ultrasensitive; Piezoresistive sensor 1 INTRODUCTION Owing to the drawbacks (e.g., limited strain detection range, rigidity and complex fabrication process) of conventional metal1 and inorganic semiconductor2 based sensors, high-performance flexible sensors have become a research hotpot with the rapid increase of flexible electronic industry, especially in the fields of electronic skin,3-8 human-machine interfaces,9-13 human


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health monitoring,14-19 speech recognition systems,20-24 etc. Currently, a variety of pressure sensors were designed according to different sensing mechanisms, including capacitive sensing,25-28 piezoelectric sensing,29-34 triboelectric sensing,35 and resistive sensing.36-40 Among them, piezoresistive sensor, which converts external compressive pressure or strain into electrical resistance variation signal, has received great attentions due to its advantages of easy processing, simple structure and low cost.31-32 Electrical conductive polymer nanocomposite (ECPCs), obtaining from the combination of electrically conductive filler with polymer matrix through suitable processing technology, has turned out to be intriguing candidates for high-performance piezoresistive sensor.41-47 Its working principle is mainly based on the deformation of ECPCs that induced the rearrangement of the conductive network upon external compressive pressure or strain, causing the variation of electrical resistance.48-50 Especially, lightweight porous ECPCs based piezoresistive sensors provided an effective solution for the defect of small strain detection range of conventional ones. Due to the high porosity and the good elasticity of polymer matrix, the “contact effect” between adjacent porous skeletons generated significant resistance variation signal over a wide strain range.31,

51-52

Recently,

our

group

has

successfully

fabricated

porous

carbon

nanotube/thermoplastic polyurethane (TPU)53 and graphene/TPU51 nanocomposites using the environment-friendly freeze-drying technique, displaying excellent piezoresistive sensing behaviors with excellent reproductivity and recoverability over 90% compression strain. In addition, commercial sponges were also proved to be ideal substrate to prepare porous piezoresistive sensor through simple dip-coating technique.54-55 For instance, porous sea sponge



was immersed into graphene and silver nanowires (AgNWs) hybrid suspension, leaving conductive layer on the sponge skeleton after drying.56 The as-prepared piezoresistive sensor exhibited high sensitivity over a detection range of 50% compression strain. But, the “contact effect” usually occurred under higher strain for these porous ECPCs discussed above, resulting a bad detectability towards small strain and limiting their special applications, such as artificial skin. To obtain porous ECPCs with good detectability for both small and large strain simultaneously, rational microstructure design appeared to be effective method for fabricating innovative porous ECPCs based piezoresistive sensors.57-58 Recently, ECPCs with special microcrack structure have shown a very positive role for enhancing its sensitivity and detectability in small strain region based on the open and closure of the cracks.57 For example, Wu et al. adopted ion sputtering technique to fabricate conductive gold coated PU sponge, and tunable crack densities were acquired through different pre-compression strain.52 As a result, the conductive sponge possessed a low detection limit (0.5 % strain), wide sensing range (55% strain) and good reproductivity. Meanwhile, graphene coated PU sponge with fractured fiber network was obtained through the hydrothermal treatment and pre-compression process, and a low pressure detection limit of 9 Pa was successfully achieved.59 However, the preparation processes of the samples stated above are complicated, which severely limit their large-scale production as piezoresistive sensors. In this study, conductive cellulose nanofibril/AgNWs (CA) coated PU (CA@PU) sponge was fabricated using simple dip-coating technique. Based on the high modulus of cellulose,60 the


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brittle conductive CA layer was easy to be fractured upon external compression, constructing cracked structure easily. In addition, the application of CNF was also beneficial for the dispersion of AgNWs and the good adhesion between the conductive layer and PU skeleton. The microstructure of the conductive CA@PU sponge was investigated using scanning electron microscope, and the compression mechanical properties were also studied. Most importantly, the piezoresistive sensing performance of the conductive CA@PU sponge under small and large compression strain were systematically tested to verify its detection limit and sensing range. Finally, the piezoresistive sensor was applied to detect human bodily motion with different strain, and a CA@PU artificial electronic skin was also designed to illustrate its broad application prospect. 2 EXPERIMENTAL SECTION 2.1 Materials Commercial PU sponge with a density of 25 Kg/m3 was purchased from Jiangxi Hongsiyuan Special Foam Material Co., Ltd. Cellulose nanofibril (CNF) aqueous dispersion with a solid content of 1 wt.% was bought from Guilin Qihong Technology Co., Ltd. Dopamine hydrochloride (DA) and tris(hydroxymethyl-amino-methane) (Tris) were purchased from Shanghai Macklin Biochemical Co., Ltd. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co., Ltd China. Poly(vinylpyrrolidone) (PVP, Mw=1300000 g mol-1) was obtained from Usolf Chemical Technology Co., Ltd, Tsingtao, China. Sodium chloride (NaCl) was bought from Zhiyuan Reagent Co., Ltd, Tianjin, China. Ethylene glycol (EG), ethyl alcohol



and acetone were supplied by Tianjin Fuyu Fine Chemical Co., Ltd, Tianjin, China. All the materials and chemicals were used as received. 2.2 Preparation of AgNWs AgNWs used in this experiment were synthesized through a modified polyol procedure61-62. Briefly, 60 mL PVP/EG solution (0.18 mol/L) and 30 μL NaCl aqueous solution (0.1 mol/L) were first added into a three-necked flask and pre-heated at 170 °C in an oil bath for 90 minutes under stirring. Subsequently, 30 mL freshly prepared AgNO3/EG solution (0.12 mol/L) was added dropwise to the three-necked flask under stirring and heated at 170 °C for another 2.5 hours. After reaction, the solution turned to be gray with silky texture and cooled to room temperature. Then, the solution was added with a large amount of acetone and centrifuged (5000 rpm/min, 10 min) for 3 times to purify the AgNWs. Finally, AgNWs was washed with ethanol for 3 times to remove excess EG and PVP and re-dispersed in water, and the mass loading of the as-prepared AgNWs dispersion was fixed as 10 mg/ml for the following application. 2.3 Fabrication of conductive CA@PU sponge As shown in Fig. 1, conductive CA@PU sponge was prepared using simple dip-coating technique. Briefly, the commercial PU was first washed with acetone and then dipped into the DA/Tris buffer solution (PH≈8.5) for 24 h at 30 oC to obtain the poly-dopamine (PDA) modified PU sponge. The color of the PU sponge turned to be brown with the deposition of PDA. Next, the PDA modified PU sponge was repeatly dipped into the CA solution (2 mg/mL) with a mass ratio of 3:2 under ultrasonication for 10 minutes. Then the excessive solution was squeezed out


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from the sponge followed by drying at 80 °C for 1 hour, achieving a gray conductive CA@PU sponge. Here, the dipping number was labeled as x, and the corresponding sample was denoted as CA@PUx.

Fig. 1 Schematic illustration for the preparation of conductive CA@PU sponge.

2.4 Preparation of piezoresistive sensor and electronic skin In this study, the prepared conductive CA@PU sponge was applied with a pre-compression treatment to produce cracked structure on the surface of sponge skeleton, during which the sponge was first compressed to a strain of 80% at a rate of 5 mm/min and then returned to the initial state using universal mechanical tester. For piezoresistive sensor, conductive silver paste was applied to the two opposite faces of cubic conductive CA@PU sponge (2×2×2 cm3) to eliminate contact resistance and to collect stable resistance signal output. Then, conductive tapes were adhered to the two faces for resistance and piezoresistive sensing test. As for electronic skin, cubic conductive CA@PU sponges (1×1×1 cm3) were used to construct a 5 pixel  5 pixel



arrays on an insulating plastic plate. The surface and bottom of each sponge was coated with conductive silver paste, then each row selection and each column selection were respectively connected by conductive tape. 2.5 Characterization The micromorphology of sponge and AgNWs were observed through scanning electron microscope (SEM, Zeiss MERLIN Compact) at an accelerating voltage of 5 KV. Transmission electron microscope (TEM) was performed with a Tecnai G2 20 instrument at an acceleration voltage of 90 kV. XRD scans in the range of 10 to 100° were recorded using a Bruker-AXS D8 Advance (Shimadzu Co. Ltd, XRD-6100) with Ni-filtered Cu-Kα radiation (λ= 0.15418 nm) at a swing speed of 0.02°/min at 40 kV. Fourier transform infrared spectroscopy (FTIR) spectrum in the range of 50 to 4000 cm-1 was collected on a Nicolet Nexus 870 instrument at a resolution of 4 cm-1 using the attenuated total reflection mode. UV-vis absorption spectroscopy was performed on a UV-vis spectrophotometer (Agilent, Cary 5000) in the range from 300 to 700 nm. Cubic CA@PU sponges (2×2×2 cm3) were subjected to a universal testing machine (SHIMADZU, Japan) at a compression rate of 5 mm/min to investigate its compression performance. The I−V curves of the piezoresistive sensors were measured by A RST5000 electrochemical workstation (Suzhou Risetest Electronic Co., Ltd, China) with linear sweep voltammetry. The resistance of the piezoresistive sensor was tested using a digital multimeter (Tektronix DMM4050). Digital multimeter and universal tensile tester were used together to measure the piezoresistive sensing behavior of the sensor online.


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3 RESULTS AND DISCUSSION 3.1 Characterizations of AgNWs

Fig. 2 (a) SEM image, (b) TEM image, (c) UV-vis spectrum and (d) XRD spectrum of the prepared AgNWs.

SEM and TEM were conducted to study the morphology of the prepared AgNWs. As shown in Fig. 2a, AgNWs with an average length of 20-30 µm were obtained with very few silver nanoparticles, showing the high purity of AgNWs. TEM image (see Fig. 2b) displayed the diameter of AgNWs is about 150 nm. Besides, about 3 nm thick PVP identified through FT-IR (see Fig. S1) was observed on the surface of AgNWs.63 It is well known that the existence of PVP can effectively prevent the oxidation of AgNWs and improve the dispersity of AgNWs in water.64 To further demonstrate the purity and the structure of AgNWs, UV-vis spectra (see Fig. 2c) and XRD spectra (see Fig. 2d) of AgNWs were characterized. Generally, the existence of the absorption peaks at 350/390 nm and 410 nm in the UV-vis spectrum are the characteristic peaks



of long AgNWs and silver nanoparticles, respectively.65-66 Hence, the absence of the absorption peak at 410 nm in Fig. 2c also verified the high purity of the prepared AgNWs. As for the XRD spectrum of AgNWs, five characteristic diffraction peaks were observed at 2θ =38.06, 44.26, 64.4, 77.34 and 81.48o, corresponding to the (111), (200), (220), (311) and (222) crystalline planes of pure face-centered cubic silver crystals.6 3.2 Characterizations of conductive CA@PU sponge

Fig. 3 SEM images of (a, b and c) PU sponge, (d, e, f) PDA treated PU sponge and (g, h, i) CA@PU sponge at different magnifications; (j) Uniform macrostructure and (k) fractured surfaces of the conductive CA@PU sponge; (l) A green leaf supports the conductive CA@PU sponge without being bent.


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As shown in Fig. 3a-c, the commercial PU sponge possessed an interconnected 3D porous structure with a pore diameter of 300-500 µm and smooth skeleton. After the self-polymerization of PDA, there was almost no obvious change about the interconnected 3D porous structure (see Fig. 3d), but the surface of the sponge skeleton become rough due to the existence of wrinkled PDA layer (see Fig. 3e). Here, PDA was applied to improve the hydrophilicity of PU sponge to facilitate the dip-coating process in the next step.55 This can be verified from Fig. S2 that the water contact angle of the PU sponge decreased from 141o to 84o, indicating the transformation of PU sponge from hydrophobicity into hydrophilicity. So it can be seen clearly in Fig. j that both the outer and fracture surfaces presented uniform gray in color, indicating the CA solution can easily permeate into the porous structure. Meanwhile, the excellent adhesion of PDA can also enhance the adhesion strength between CA layer and PU skeleton.67 After cyclic dip-coating in CA solution, the conductive CA@PU sponge also inherited the interconnected 3D porous structure of PU (see Fig. 3g), but the roughness of the surface of the porous skeleton was further increased due to the homogeneous distribution of AgNWs (see Fig. 3h). Besides, as expected, cracked CA conductive layer was successfully constructed on the sponge skeleton, Fig. 3i. Finally, a green leaf could support the conductive CA@PU sponge without being bent (see Fig. K), indicating the lightweight characteristic of PU sponge was well maintained. 3.3 Mechanical property of conductive CA@PU sponge



Fig. 4 Compressive stress-strain curves of (a) PU sponge and (b) conductive CA@PU24 sponge under different strains. The inset in (b) is the digital pictures showing the ultrahigh compressibility and good recoverability of conductive CA@PU sponge; (c) Compressive strength and compressive modulus of different CA@PU sponges; (d) Stress-strain curves of CA@PU24 under 40% strain for the 1th and 500th cycle.

Generally, good mechanical property is crucial for ideal piezoresistive sensor. Here, a series of compression tests were conducted at a compression rate of 5 mm/min to study the mechanical property of PU sponge and different CA@PU sponges. As shown in Fig. 4a, three typical distinct stages were observed for the compressive stress-strain curves of PU sponge:68 the linear elastic region for ε

Ultrasensitive and Highly Compressible Piezoresistive Sensor Based

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